A CRT (cathode ray tube) has been mainly employed in various display devices used for office automation (OA) equipment such as a word processor, a notebook-sized personal computer and a personal computer monitor, mobile phone terminal and television set. In recent years, a liquid crystal display device has more widely been used in place of a CRT, because of its thinness, lightweight and low power consumption. A liquid crystal display device usually comprises a liquid crystal cell and polarizing plates. The polarizing plate usually has protective films and a polarizing film, and is obtained typically by dying a polarizing film composed of a polyvinyl alcohol film with iodine, stretching the film, and laminating the film with the protective films on both surfaces. A transmissive liquid crystal display device usually comprises polarizing plates on both sides of a liquid crystal cell, and occasionally comprises one or more optical compensation films. A reflective liquid crystal display device usually comprises a reflector plate, a liquid crystal cell, one or more optical compensation films, and a polarizing plate in this order. A liquid crystal cell comprises liquid-crystalline molecules, two substrates encapsulating the liquid-crystalline molecules, and electrode layers applying voltage to the liquid-crystalline molecules. The liquid crystal cell switches ON and OFF displays depending on variation in orientation state of the liquid-crystalline molecules, and is applicable both to transmission type and reflective type, of which display modes ever proposed include TN (twisted nematic), IPS (in-plane switching), OCB (optically compensatory bend) and VA (vertically aligned) ECB (electrically controlled birefringence), and STN (super twisted nematic). Color and contrast displayed by the conventional liquid crystal display device, however, vary depending on the viewing angle. Therefore, it cannot be said that the viewing angle characteristics of the liquid crystal display device is superior to those of the CRT.
In order to improve the viewing angle characteristics, retardation plates for viewing-angle optical compensation, or, in other words, optical compensation sheets, have been used. There have been proposed various LCDs, employing a mode and an optical compensation sheet having an appropriate optical property for the mode, excellent in contrast characteristics without dependency on viewing angles. An OCB, VA or IPS modes are known as a wide-viewing mode, and LCDs employing such a mode can give a good contrast characteristic in all around view, and, then, become widely used as a home screen such as TV. Further, in recent years, a wide screen of over 30 inches has been also proposed.
An optical compensation sheet can effectively contribute to reducing the viewing angle dependence of contrast, but cannot contribute to reducing the viewing angle dependence of color sufficiently, and reducing the viewing angle dependence of color is considered as an important problem to be solved for LCD. Viewing angle dependence of color of LCD is ascribable to difference in wavelength of three representative colors of R, G and B, so that even R, G and B lights go through are given equal retardation, the changes in polarization states of R, G and B lights brought about by the retardation are different each other. In view of optimizing this, it is necessary to optimize wavelength dispersion of birefringence of an optically anisotropic material with respect to the wavelengths of R, G and B. The LCD is, however, still on the way to thorough improvement in the viewing angle dependence of color, because it is still not easy to control the wavelength dispersion of birefringence of liquid crystal molecules used for ON/OFF display, or for optical compensation sheet.
There has been proposed a retardation plate using a modified polycarbonate, as an optical compensation sheet controlled in the wavelength dispersion of birefringence for reducing viewing angle dependence of color (Japanese Laid-Open Patent Publication “Tokkai” No. 2004-37837). Viewing angle dependence of color can be reduced by using this plate as a λ/4 plate for reflection-type liquid crystal display device, or as a compensation sheet for VA-mode device. It has, however, not been widely used yet for LCD, not only because the modified polycarbonate film is expensive, but also because the film tends to cause non-uniformity in the optical characteristics such as bowing during stretching included in the process of producing them.
On the other hand, based on the same principle as that of the viewing-angle compensation of contrast using the optical compensation sheet, a system has been also proposed which compensates the wavelength dispersion independently for each of three colors of R, G and B (GB2394718). The optical compensation is achieved mainly by patterning of a retardation plate together with a color filter or the like inside of a liquid crystal cell. However, it was difficult to form an optically anisotropic layer having a uniform retardation characteristic inside of a liquid crystal cell by using a patternable material.